Chassis, generic

C05CE Brake Master Cylinder Piston Position Sensor B Circuit Voltage Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Here the second, PWM-coded output (channel B) of the piston travel sensor stays permanently low. GM's XT4 brake module and the later 23OBDG07 and 25OBDG03B tables interpret a constant low level for 0.2 s as the signal line shorted to ground, Type A, one trip, MIL. Because a PWM line at 0 V also results when the sensor loses its supply or ground, a dead channel B together with a working channel A suggests a wiring fault close to the second output, while both channels failing hints at the shared supply (C05D1). The module can usually still brake with channel A, but with less redundancy. Inspect the harness at the booster for crush or water damage before condemning the unit. C05CD is the stuck-high case.

Likely causes

PWM signal line shorted to ground
Sensor channel B without supply or ground
Output stage of channel B failed low

Symptoms

  • Malfunction indicator lamp, brake system warning
  • Possibly reduced assist or regeneration off

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–2,000
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

master cylinder pos sensor bsensor wiring

Related codes

C05CDC05CFC05CB

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.